I know that this should come easy as adding fans is one of the basics of modding, but instruction as to how to make circles and cut them seems to be lacking (at least from Modders-Inc). I thought that I would write up a guide.
There are two parts to modding: planning and execution. You are only as good as your weakest link, and in this two-linked chain that old adage is especially true. Even if you are a master of the scroll saw, a misguided cut can mean disaster. Conversely, if you draw up the best plan in the world and mess it up beyond all recognition, equal disaster will result.
Anyway, on to the guide.
First: TapeTapeTape! Use blue tape if you plan on leaving it on for more than a day or so. If you're using a scroll saw you'll also want to drill out the rivets to detach the panel that you're cutting into. Only tape painted sides and if youplan on repainting then tape shouldn't be necessary. I didn't tape in my example because I needed to repaint. This is why you want to tape:
All those little specks are evil paint-scratching metal shavings.
Second you want to get your location determined and your screw holes marked out. Get yourself an average-quality pen with a removable center. You can't use a really nice one because the ink resevoire is too thick to fit through the hole, and the resevoire detaches from the ball part of really cheap ones. Here's a pic of the three varieties in descending cheapness:
You want the one in the middle.
Next, place your fan exactly where you want it. Shove the pen core inside the holes and wiggle.
Then, find the distance between the holes diagonally.
Here the distance is 4". I remarked the holes with sharpie for this guide because the pen marks weren't showing up on the camera, but the ball-point is more accurate so in real life I would just leave it in pen.
Find the bisector of that length (2" and transfer it to your compass.
Now, find your Centaur!
Using a screw hole as your axes, rotate your way through the center. Then do this from each screw mark. The marks should all come together around the center. This is your center (or centaur).
Now, before you make the final circle, we need to take a slight pause to learn another important saying: Perfection before Convention! The computer industry is littered with naming convention misnomers. For instance, 5.25" drives are 5.75", and 3.5" drives are 4". Always measure twice, never assume. The circular part of an 80mm fan is typically 76mm in diameter, so our radius is 38mm.
Drawing the circle:
These pics clarify what I was talking about with finding your centaur.
When making the cut, make sure that you use a sharp blade, especially when cutting steel as I am doing today. Also, make sure that the blade is lubricated. I prefer regular unsalted butter for lubrication as it stays on longer than oil, but find what works for you. It is difficult to keep the balance between pushing and turning. Obviously, the center of your circle is the axes of your cut, so you can put your thumb in the center of your circle and rotate your workpiece around it. When drilling the hole to feed the blade through, keep it as close to the edge of your cut as possible without going outside the line. Before you start cutting, decide whether you're going to cut on the inside or the outside of the line. It is very important to stay consistent. I like to try to cut the inside of the line, so if I accidentally cut outside the line I can go back around and cut the outside of the line.
On this particular cut I learned the perfection before convention part, so I cut it too big and don't want to mess you guys up, so no pics. I did make another cut just like this one directly below it, and on this one I used all of my skills.
Kazow! Perfect! Never waste your money on grilles again!
Don't expect your first one to be perfect, and filing of some sort is almost always necessary.
I hope that this helps some people, because when I first started I had no idea how to place and cut a blowhole. If anyone has anything to add or think that I forgot something let me know and I'll change it.